In North American wireless cellular telephony using a time divisional multiple access (TDMA) communication protocol, a voice signal in either traffic direction, that is, base station downstream to mobile station, or mobile station upstream to base station, is represented as a sequence of digitized speech frames. Each digitized speech frame is a block of a predetermined number of binary digits, representing the output of a speech-compressing analog-to-digital converter, together with various binary check digits and coding bits used for error detection and error correction.
To provide privacy in conversations, a transmitting station using a conventional encryption technique forms a fixed voice privacy mask (FVPM) having the same predetermined number of binary digits as the speech frame, and encrypts each frame with this particular FVPM, typically by simply combining the speech frame and the FVPM using an exclusive-or (XOR) operation, bit by bit. Decryption is performed at the receiving station by combining the received speech frame and the FVPM, again typically by XOR-ing the received frame and the FVPM. Since XOR-ing corresponds to inverting a bit, it will be appreciated that doubly inverting a binary digit recovers its initial value.
An advantage of this conventional technique is that the transmitting station and receiving station each have a procedure for privately generating the FVPM, so the FVPM is not transmitted and is not directly available to eavesdroppers. This technique is suitable for protecting against unintentional eavesdropping, such as incorrect channel tuning, or a simple channel scanning operation. Real-time decryption without knowledge of the FVPM is difficult using readily available computing systems.
A disadvantage of this conventional technique is that it does not provide sufficient security. More particularly, if an encrypted conversation is recorded, subsequent analysis can fairly easily determine the value of the FVPM and then perform unauthorized decryption of the conversation. Furthermore, it is believed to be only a matter of time until computing systems become readily available for real-time unauthorized decryption of conversations encrypted according to this conventional technique.
Thus, there is a need for an encryption technique wherein the transmitting station and receiving station each have a procedure for privately generating the encryption mask for transmitted frames, and which increases the difficulty of unauthorized decryption.